A GeV Gamma-Ray Spectral Excess in the Fermi Blazar 4FGL J0136.5+3906

We report the detection of a $\sim${19} GeV gamma-ray spectral excess (peak energy in observer frame) at $\sim$5.7$\sigma$ significance (3 degrees of freedom, without correction for trials and the look-elsewhere effect) in the Fermi-LAT data of the blazar 4FGL J0136.5$+$3906, based on $\sim${17} years of observations. The excess manifests as a distinct bump-like feature in the 100 MeV$-$1 TeV spectrum, with a flux of $\sim$3.6$\times {10}^{-10}\mathrm{p}\mathrm{h}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$. No similar feature is detected in nearby sources or background regions. The origin of this signal cannot be explained by standard blazar emission models and remains unknown. This Letter presents the observational finding without providing a definitive physical interpretation.